Reticuloendotheliosis virus (REV-T) induces immature lymphoid tumors in the chicken. The ability of REV-T to induce tumors in different types of lymphoid tissues will be examined. The phenotype of the REV-T induced tumor is consistent with the hypothesis that tumor development is accompanied by inhibition of lymphoid differentiation. Experiments will evaluate this hypothesis and also determine whether inactivation of the protein responsible for tumor development, v-rel, is accompanied by the reinitiation of lymphoid differentiation. These experiments will require the use of a helper-free REV-T. Purification of specific populations of lymphoid cells will be infected in vitro with this virus and injected into recipient animals for in vivo tumor development. The phenotypes of the target cell and the tumor cell will be compared to determine their relationship. A mutant of REV-T that is temperature sensitive for functional v-rel will be prepared in order to study the effect of v-rel inactivation on lymphoid differentiation.